The present invention relates to data communication systems and, more particularly, to data communication systems that are operable in more than one mode. Data communication systems of this sort frequently utilize the public switched telephone network for at least a portion of the communication channel.
At the endpoints of a data communication system, one will find data communication equipment. The primary element of such equipment is typically some sort of modem. A communication channel connects the endpoints so that information may be communicated between the endpoints. The communication channel may be analog, digital or, commonly, a combination of digital and analog links. The modems typically modulate information for transmission over the communication channel and demodulate signals received from the communication channel to recover information.
A schematic representation of a data communication system is shown in FIG. 1. A first modem 10 is connected to a second modem 20 by a communication channel 30. The first modem 10 modulates input data 40 for transmission over the channel 30 to the second modem 20. The second modem 20 recovers the input data 40 by demodulating the signal received from the first modem 20. In like manner, input data 50 may be modulated by the second modem 20 and transmitted over the channel 30 to the first modem 10.
Implicit in the foregoing are: 1) there are different types of modems; and 2) different types of modems should be capable of communicating with each other. With respect to the different types of modems, modems may be classified, for example, by manufacturer, by structure (e.g. internal, external, PC card etc . . . ) or by the communication protocols that they support. For purposes of the present invention, the third criterion is the most pertinent.
A communication protocol, generally speaking, is a set of rules that defines how data communication devices interact. Numerous communication protocols have been developed to enable data communication between a wide range of data communication devices. For accurate information transfer between devices, the devices must operate under, or support, the same protocol.
It is becoming increasingly common for data communication devices to support several communication protocols. An appropriate communication protocol may be selected from among the available protocols depending upon the capabilities of the remote device and the communication channel. Unfortunately, one typically does not know, before a connection is made over the public switched telephone network, what type of modem will be encountered at the remote end of a switched connection. It is therefore desirable, at an early stage in the connection process, to identify the communication protocols that are supported. In addition, it is usually desirable to communicate information at the highest rate that the modems and the communication channel can reliably support.
The International Telecommunication Union-Telecommunications Standardization Bureau ("ITU-T") in Geneva, Switzerland, develops and publishes "Recommendations" that relate to communication protocols. The ITU-T Recommendations are non-binding international standards whose objective is to ensure compatibility of international telecommunications on a world-wide basis. The Recommendations referred to herein are publicly available from the ITU-T.
Protocols are known that identify the capabilities of a call modem that is connected to an answer modem by a communication channel. For example, Recommendation V.34, which is incorporated herein by reference and which is entitled A Modem Operating at Data Signalling Rates of up to 28,800 bit/s for use on the General Switched Telephone Network and on Leased Point-to-Point 2-Wire Telephone-Type Circuits, describes a network interaction protocol for identifying V.34 capability.
The V.34 protocol includes a preliminary data exchange according to ITU-T Recommendation V.8, Procedures for Starting Sessions of Data Transmission over the General Switched Telephone Network. Recommendation V.8 is incorporated herein by reference. During the preliminary data exchange, a call modem initially conditions its receiver to detect an answer tone, such as ANS or ANSam, from an answer modem. The call modem then transmits a call indicator or call tone, such as CI, CNG or CT, as defined by Recommendation V.8.
The ANS signal is a 2100 Hz tone, and is the typical tone produced by an answering modem that is not V.8 compatible. However, if the answering modem is V.8compatible, it provides the ANSam signal, which is defined as a 2100 Hz tone that has been amplitude modulated with a 15 Hz tone.
If the signal response ANSam is detected by the call modem, then the call modem transmits silence for a period of time, T.sub.e as specified by Recommendation V.8. If, on the other hand, the signal response ANS is detected, indicating that the answer modem is incapable of performing the CM/JM exchange, the call modem proceeds under a different protocol, for example as provided in Annex A of Recommendation V.32bis or Recommendation T.30.
Next, the call modem conditions its receiver to detect a joint menu signal, JM, and transmits a call menu signal, CM. The call menu signal, CM, includes the appropriate bits set in the modulation modes category, as provided in Table 4/V.8, to indicate that operation in accordance with Recommendation V.34 is desired. The CM sequence includes eight information bits (an octet) plus a start bit and a stop bit, for a total of ten bits per word.
When a minimum of two identical JM sequences have been received, the call modem completes the current CM octet and then sends a call menu terminator signal, CJ, to acknowledge JM and terminates the call menu signal. After sending CJ, the call modem transmits silence for 75.+-.5 ms and proceeds with the remaining steps of ranging/probing, equalizer and echo-canceller training, and final training as described in Recommendation V.34.
During the Recommendation V.8 interchange described above, the CM and JM words are transmitted between modems using the signaling format specified in Recommendation V.21, which is entitled 300 Bits Per Second Duplex Modem Standardized for Use in the General Switched Telephone Network. Recommendation V.21, which is incorporated herein by reference, defines a type of Frequency Shift Keying (FSK) that is referred to herein as Frequency Division Modulation (FDM). The modulation format operates at 300 bits per second (bps), where a logic one bit (also referred to as a "mark") is transmitted from the call modem to the answer modem (channel no. 1) as a 980 Hz nominal tone, and a logic zero bit (also known as a "space") is transmitted from the call modem to the answer modem (channel no. 1) as a 1180 Hz nominal tone. Recommendation V.21 specifies that the transmitter frequencies are to be within .+-.6 Hz of their nominal values, and that the receiver must be able to correctly interpret the mark and space frequencies if they are within 12 Hz of their nominal values.
A disadvantage of the network interaction protocol described in Recommendation V.34 is that it does not identify the availability of non-standard or later devised data signalling modes or protocols. It is desirable to identify the availability of non-standard or later devised capabilities. In addition, it is desirable to do so without interfering with normal training and operation under Recommendation V.34.
Accordingly it would be desirable to have an improved mode signalling method and apparatus.